1) Field of the Invention
The present invention relates to an image forming apparatus such as a printer, a copying machine, a facsimile, and a plotter. More specifically, this invention relates to a cleaning member used in the image forming apparatus, a cleaning device using the cleaning member, and a charging device, a transfer device, a process cartridge, and the image forming apparatus each including the cleaning device.
2) Description of the Related Art
Image forming apparatuses such as printers, copying machines, facsimiles, plotters, and those using electro-photography to form images, are widely used. Such an image forming apparatus forms images in the following manner. The image forming apparatus includes an image carrier that is an electro-photographic photosensitive element (photoconductive element). A charging device charges the image carrier, and an optical write operation is performed on the charged image carrier, to form an electrostatic latent image thereon. A developing device develops the electrostatic latent image with a developer, to visualize as a visible image. A transfer unit, directly or through an intermediate transfer element, transfers the visible image on the image carrier to a transfer material. The visible image on the transfer material is fixed to form an image thereon.
Attempts are being made to increase the life of the electro photographic image forming apparatuses. To achieve this goal, it is attempted to prevent reduction in film thickness of the image carrier, which is disposed at a central position of the image forming apparatus, and to prevent soiling of the charging device and the transfer unit, to thereby ensure excellent image formation over a longer duration. Consequently, running costs largely reduce.
It is known that the charging device largely affects the reduction in film thickness of the image carrier. There are various types of charging devices such as contact charging, and “micro-gap” charging (proximity charging) that uses a micro-space formed for charging. These charging devices may be constructed as a charge roller made of an elastic material with its resistance adjusted in the following manner. The charge roller is provided in contact with or close to the image carrier, and applies charging bias to the image carrier while the charge roller is made to rotate along with the image carrier. In such case, an alternating current (ac) voltage is superimposed on a direct current (dc) voltage, and the superimposed voltages are applied as the charging bias. The ac voltage has a peak-to-peak voltage twice as high as that of a discharge start voltage of the dc voltage. Application of the ac voltage causes the potential on the surface of the image carrier to converge to a value of the dc voltage applied, which allows uniform charging over the surface of the image carrier.
However, the contact charging or the proximity charging is carried out by charging the surface of the image carrier by pulsed discharge generated in a micro-space between the image carrier and the charge roller. Therefore, the surface of the image carrier is etched all the time. Consequently, there is a further reduction in film thickness of the image carrier.
Some new charging methods as follows are proposed, to suppress the reduction in film thickness of the image carrier.
(1) DC Charging (Method of applying only a dc voltage to a contact type charge roller):
In this method, the surface of the image carrier is charged only with the dc voltage. Because the ac voltage is not applied thereto, the amount of current flowing in the image carrier is extremely small. In other words, pulsed discharge to the image carrier also decreases. Consequently, the etching of the image carrier (photosensitive element) reduces, and the reduction in film thickness of the image carrier is less.
(2) Coating of Lubricant to Image Carrier:
A lubricant layer is formed on the image carrier using a cleaning device for the image carrier, or a solid lubricant coating device provided as a discrete unit, and the reduction in film thickness of the image carrier is suppressed. Zinc stearate is a typical solid lubricant.
However, these charging methods have the following two problems:    (1) In the dc charging, soil (flying toner upon transfer, paper dust, and the like) is easily deposited on the surface of the charge roller. Therefore, electrical resistance and the state of the surface change, thereby inhibiting uniform charging. The reason is that, because the ac voltage is not applied, even a slight fluctuation in resistance results in a soiled image and nonuniform density.    (2) When the solid lubricant is coated on the image carrier, cleaning capability is very stable even under environmental fluctuations of the image carrier. This is because a frictional coefficient with the cleaning unit, typified by a cleaning blade, is low and stable. However, the low frictional coefficient increases the amount of toner particles, which are escaping through the blade little by little (toner particles passing through the blade). In other words, by making life of the image carrier longer, the toner particles escaping from the cleaning device increase, which further soils the charging device.
Many imaging units of the image forming apparatuses in recent years are configured to have a process cartridge because of easy replacement and maintenance. However, because the process cartridge includes the image carrier and the devices around the image carrier integrated into one unit, improvement in life of the image carrier only does not necessarily lead to improvement in life of the process cartridge.
In other words, some measures are required to improve the life of not only the image carrier, but also the devices around the image carrier.
Furthermore, image forming apparatuses such as printers and copying machines that employ polymer toner are available in the market due to demand for higher image quality. However, the polymer toner is generally difficult to clean, and therefore, it is more difficult to achieve the increase in life of the devices than in case of the conventional pulverized toner.
Japanese Patent Application Laid Open No. 2002-221874 and Japanese Patent Application Laid Open No. 2003-66807 disclose technologies in which, melamine resin foam that is made of melamine foam is used as a cleaning member, to remove residual toner on the transfer member and the paper dust. Therefore, it is described that using the melamine resin foam is an excellent method to increase the life of the devices.
The melamine foam has a three-dimensional mesh structure in which cells of some tens to hundreds of micrometers are linked. Small-sized foreign matter is attracted to the surface of the mesh, and more foreign matter can be accommodated therein because of spatial linkage. Therefore, the melamine foam is useful as a cleaning member for the electro-photographic image forming apparatuses.
However, in the melamine resin foam that foams the melamine foam, a large spherical cell having a diameter of about a few millimeters (mm) is produced in some rare cases. It is called “pinhole”. The portion where the pinhole is present does not naturally contact a member to be cleaned, and therefore, the melamine foam having the pinhole does not serve the function of cleaning. It is therefore regarded that the melamine foam is not suitable for the cleaning member, and is abandoned. When the cleaning member is to be formed, a portion is cut out into a required size from a large block of melamine resin foam, is bonded to a support by an adhesive, and then cutting and polishing are performed thereon for finishing. Thus, the presence of the pinhole in the contact plane of the cleaning member can not be determined before cutting and polishing. Moreover, abandoning the cleaning member results in a big demerit of cost.
The melamine resin foam thus foamed has fluctuations in a foaming factor, and therefore, there are many pinholes on the surface thereof, each having a diameter of about 1 to 3 mm. Moreover, when the foam is cut and formed as the cleaning member, there are pinholes larger than these. A hole having a diameter of 3 mm or more can be checked based on whether the foam with the hole is a qualified component. However, there are many pinholes having a diameter of about 1 to 3 mm. If these foams are determined as faulty components, yield is reduced significantly. It is thus impossible to obtain non-defective components (it is quite clear if you look at the surface of a sponge for household use made of the melamine resin foam).
The presence of the pinholes reduces a contact area with a target cleaning member, and partial reduction in cleaning capability results in nonuniform charging and nonuniform transfer. Particularly, the nonuniform charging and nonuniform transfer result in uneven color in color electro-photography, which is a vital defect.